Fail-fast behavior (structured cancellation)

By default, if a task fails and you await it:

• the error propagates normally
• sibling tasks in the same nursery should be cancelled (fail-fast)

Typical pattern:

var {*} = import("slug.channel")
nursery fn handler(req) {
    var aT = spawn { taskA() }
    var bT = spawn { taskB() }

    var a = await(aT);    // if this throws, nursery cancels siblings
    var b = await(bT)

    {a:a, b:b}
}